1. Field of the Invention
This invention relates to a method for catalytic gas phase oxidation adapted to introduce a heating medium into a reactor for catalytic gas phase oxidation by means of a circulation device and characterized by that the introduction of the heating medium being cooled and then circulated into the reactor is effected in the proximity of the inlet to a heating medium circulation system equipped with the reactor or the outlet side of an annular conduit surrounding the reactor and the amount of the heating medium to be circulated is a prescribed quantity.
2. Description of Related Art
The reaction of catalytic gas phase oxidation using a shell-and-tube type reactor is a means generally used for efficiently removing the heat which is generated by the reaction. The shell-and-tube type reactor used herein has a plurality of reaction tubes built in the shell of the reactor. The reaction tubes packed in advance with a catalyst are supplied with the raw material gas for the reaction and caused to attain the reaction of catalytic gas phase oxidation and the shell of the reactor is meanwhile caused to circulate therein a heating medium capable of absorbing the heat of reaction and absorb the produced heat of reaction.
The reaction of catalytic gas phase oxidation using the shell-and-tube type reactor constructed as described above, however, tends to form a hot spot on the raw material inlet side of the tubes and, consequently entails the problem that the catalyst packed into the tubes is damaged and the selectivity of a target product is lowered by an ensuing excessive exothermic reaction. In the case of producing acrolein from propene by the reaction of catalytic gas phase oxidation, for example, how efficiently the heat of reaction is removed with a view to improving the yield of the target product constitutes itself a problem.
The official gazette of, U.S. Pat. No. 3,871,445, for example, discloses a shell-and-tube type reaction apparatus provided with a device for circulating a heat exchange medium and characterized by having a baffle plate disposed in the shell of a reactor and further having at least one annular conduit disposed at one intermediate portion at least for the purpose of leading in or leading out part of the heat exchange medium. Owing to this disposition of the baffle plate, the lateral flow velocity of the heat exchange medium is retained at a fixed level and the transfer of heat is fixed as well in the space intervening between the baffle plates. This apparatus uses the heat exchange medium by circulating it inside and outside the reactor. The heat exchange medium discharged from the reactor is cooled by a cooling device built in a circulation device and then introduced into the reactor.
The official gazette of U.S. Pat. No. 4,657,741 discloses a shell-and-tube type reaction apparatus having a device for circulation of a heating medium connected thereto and it has a plurality of circulation devices connected thereto. It is aimed at reducing the cost of construction concerning equipment, and space while accomplishing manufacture of a reaction product of uniform and fine quality in a high yield and, as asserted in the specification, is expected to warrant thoroughly uniform inflow and outflow of the heating medium by the use of a plurality of pumps and allow a prominent decrease in the requirement of output by shortening the maximum feed distance.
In addition thereto, the official gazette of U.S. Pat. No. 5,821,390 discloses a method for the catalytic gas phase oxidation of propene to acrolein and the simultaneous retention of specific selectivity and conversion rate by the use of a shell-and-tube type reactor, which method, by guiding a heating medium in a cocurrent flow and, at the same time, having a baffle plate disposed in the reactor, attains adjustment of the flow rate of the heating medium so that the temperature of the heating medium within the reactor increases in the range of 2-10xc2x0 C. According to the method disclosed in the official gazette, the acrolein is obtained by subjecting the propene to catalytic gas phase oxidation using a complex metal oxide catalytically active at a temperature elevated in the shell-and-tube type reactor and meanwhile reducing the temperature of the hot spot.
When the heating medium introduced into the shell side of the reactor for the purpose of removing the heat of reaction brings no sufficient removal of the heat of reaction, the possibility that the temperature will rise excessively in the catalyst layer to increase a side reaction, lower the yield, promote the damage of the catalyst, and induce a runaway reaction is not denied. In this case, giving a decreased diameter and an increased heat transfer area to the reaction tubes as a measure for efficient removal of the heat of reaction results in increasing the number of reaction tubes thereby heightening the cost of the reactors.
The heat balance is computed by the balance between the amount of heat generated by the reaction and the amount of heat consumed by cooling. Even the apparatus which is disclosed in the official gazette of U.S. Pat. No. 3,871,445 ought to allow more efficient removal of heat, depending on the conditions of use thereof.
The invention disclosed in the official gazette of U.S. Pat. No. 4,657,741 contemplates providing a plurality of circulation pumps for ensuring uniform circulation of a heating medium. Solely with the pressure exerted by the circulating pump, the inherently uniform mixture will not be easily attained. The removal of heat with an excellent heat exchange ratio, therefore, will not be easily attained unless the heating medium to be circulated has a uniform temperature distribution before it is introduced into the reactor by the pump.
As is clear from the working example cited in the official gazette of U.S. Pat. No. 5,821,390, under the conditions which fix the amount of the heat to be generated, the power of the pump which is required for limiting the rise of the temperature of the heating medium to 1xc2x0 C. is perfectly identical no matter whether the heating medium is fed in counter flow or in cocurrent flow inside the shell of the reactor.
The temperature of the hot spot generated in the reaction tubes have possibility to induce such things as deterioration of the catalyst. Particularly, the portion that reaches the highest temperature in all the reaction tubes tends to determine the rate of the removal of heat. As a result, the question how uniformly the heat of the reaction tubes is removed and the maximum value of the hot spot is reduced emerges as a problem. None of the techniques of prior art, however, has been capable of accomplishing the thorough removal of the heat.
Since the amount of heat to be removed is determined by the relation thereof with the amount of heat to be generated, the desirability of developing the most efficient method for removing the heat of the reaction without damaging the yield of the production for a fixed apparatus under fixed reaction conditions has been finding growing recognition.
The present inventor, in consequence of a study pursued concerning methods for the circulation of a heating medium in a shell-and-tube type reactor, has discovered that when the heating medium extracted from the shell of the reactor is circulated to a specific place in a circulation device, the heating medium which has undergone heat exchange and the heating medium which has been discharged from the shell of the reactor are mixed exceptionally efficiently. This invention has been perfected as a result. That is, the object mentioned above is accomplished as follows.
In the reaction of catalytic gas phase oxidation by means of a shell-and-tube type reactor adapted to circulate a heating medium to the shell of the reactor through the medium of a circulation device connecting an annular conduit connected thereto, this invention concerns a method for the catalytic gas phase oxidation characterized by subjecting a part of the heating medium extracted from the shell of the reactor to heat exchange, introducing the heating medium resulting from the heat exchange into the proximity of a heating medium circulation inlet on the inlet side of the circulation device or the annular conduit on the outlet side of the reactor, setting the flow rate of the heating medium after the heat exchange in the range of 2-40 vol. % based on the flow rate of the heating medium within the shell of the reactor, and setting the temperature difference between the heating medium after the heat exchange and the heating medium being introduced into the shell of the reactor in the range of 15-150xc2x0 C.
This invention is capable of reducing the hot spot temperature because it enables the heating medium 10a (shown FIG. 1) being circulated within the reactor to be mixed very easily with the heating medium 10b (shown FIG. 1) which has been cooled and consequently allows the heating medium having uniform heat distribution to be introduced into the shell of the reactor by causing the heating medium which has been cooled and is ready for circulation to the shell-and-tube type reactor to be supplied to the position confronting the inlet to the heating medium circulation system in the circulation device or to the annular conduit through which the heating medium is extracted from the shell-and-tube reactor.
The above and other objects, features, and advantages of the present invention will become clear from the following description of the preferred embodiments.